To those of you who are unfamiliar with me: Thank you for coming, and welcome to my story! For those of you who do know me...yeah, I know, I have about a million other projects I should be finishing, blah blah...but I couldn't let this idea slip away!

If you're here, I hope you are familiar with Phineas and Ferb, and at least the TMNT movie Turtles Forever. If not, I suggest you watch the movies of both or at least the first episode of both series before continuing. Thank you.

Now...see, I got this idea when I realized both shows had some kind of dimension-hops in them. Phineas and Ferb Across the Second Dimension has the Alternate-Dimension-inator; and Turtles Forever has the trans-dimensional portal, and the portal stick. I thought it would be interesting to see devices similar to these being used to have the two shows meet...and thus, this was born!

I hope you guys are prepared...for a collision of universes!

Disclaimer: I do not own Phineas and Ferb nor TMNT. TMNT belongs to Kevin Eastman and Peter Laird; Phineas and Ferb belong to Jeff "Swampy" Marsh and Dan Povenmire.

Collision

The Trans-Dimensional Problem

"There! It's done!" The red-haired boy with a triangular head stepped back, gazing upon his completed work proudly. His green-haired step-brother put a hand on his shoulder, to show that he, too, was proud of their accomplishment.

"Um…Phineas, what exactly does this thing…do?" This question came from the only female present at the moment, the black-haired Mexican-Jewish girl Isabella. Phineas smiled.

"Well, Isabella, it's a trans-dimensional portal!" Phineas explained. "You see…"

As he went on to explain a few of the finer workings of the machine, the largest boy, Buford, glanced at the Indian nerd Baljeet. "Do you have any idea what they're talking about?"

"Yes, now please, be quiet, I would like to listen!"

"So in short, this device allows us to open windows to other places!" Phineas was finishing.

"Wow," Isabella said, obviously impressed. "So…we'll finally be able to see where those giant floating baby heads come from?"

"I sure hope so," Phineas replied, glancing around in spite of himself. "Those things are really creepy sometimes." As if to prove his point, a random, giant, floating baby head floated by; the group stared at it, awestruck.

"So anyway," the red-head went on, shaking his head to dismiss the random occurrence, "let's fire it up! Oh, hey, where's Perry?"

In an underground lair just below their feet, the teal-colored platypus dropped down from a chute, a fedora landing on his head. On the screen was a white-haired man.

"Ah, good morning, Agent P," Major Monogram greeted. "Now…ordinarily, we'd have you go after Dr. Doofenshmirtz, but he's sick today, and has not purchased any weird items. We have a few undercover agents staking out his building, just in case he does try something, but we have far bigger matters at hand right now.

"Your boys have built a trans-dimensional portal, similar to the Alternate-Dimension-inator Doofenshmirtz built a few weeks ago. We don't know if this is just coincidence, or a side effect of the Amnesia-inator, but if they accidentally open up into the second dimension, they are very apt to remember everything, and then…we'd have to relocate you, Agent P."

Perry's face fell. He loved his boys, and that day was one of the best days they'd had together; Phineas and Ferb had found out about his secret identity, but after some bumps in the road they had really bonded that day. And it was all gone, just because OWCA's security precautions wouldn't allow for his family to remember. But even with those memories gone, he would not stand for being relocated.

"Your mission for today," Monogram continued, "is simply to make sure the boys don't remember, and don't open a window into that dimension. Our organization rests in your hands, Agent P."

Perry saluted, and then rushed over to the tube that would take him back up to his boys.

"So, ready…set…oh, there you are, Perry!" The platypus made his chatter as the group all looked over to him. "OK, again…ready…set…"

"Hold it right there!" Candace, Phineas and Ferb's older sister, stormed out of the house, blue eyes narrowed dangerously. "Alright, what have you two built this time?"

"It's a trans-dimensional portal," Phineas explained to his sister."We're going to use it to take a peek at some other dimensions, see what they're like. Alright, enough countdowns now; fire it up, Ferb!"

Ferb gave a thumbs up, and then hit a button. The machine whirred and sputtered, and the group gasped as a window began to form…

In another dimension…

"Donny, would you stop fooling around with the portal stick?" The blue-banded, katana-wielding Turtle was glaring at his purple-masked brother. The Turtle in question turned around, raising a skeptical eye-ridge at his older brother.

"Leo, I'm just testing to see what a few other dimensions are like," Donatello explained. Their two brothers, Raphael and Michelangelo, padded over.

"Hey Donny, do you think there are dimensions that don't have Turtles?" Michelangelo asked. This earned him a glare from Leonardo, but the orange-masked Turtle continued, "I mean, we exist in this 'Turtle-verse', but I wonder if there are other dimensions out there, you know?"

"Hmm…" Don's chocolate eyes became thoughtful. "It's possible… let me change a few of the calibrations, and we'll find out."

"Just don't do anything dangerous, Brainiac," Raphael muttered, leaning over his brother's shoulder. "I don't wanna have a horde of…of…of giant floating baby heads take over NYC, for example."

The red-masked Turtle then noticed the looks he was receiving from his brothers. "What?"

Leo rolled his eyes. "Alright, whatever, just…do you know what you're doing, Donny?"

"Yeah, Don, I don't really want any giant floating baby heads in our lair," Mikey added. This earned him a slap on the back of the head from Raph. "Ow! What?"

By this point, Don had pushed the button to activate the trans-dimensional portal stick. There was a whirring sound, and a bright flash filled Don's lab. The Turtles gasped.

"Don…where does it lead to?" Leo asked, his voice just above a whisper.

"I'm…not sure," Don replied, equally quiet. "But…"

The purple-masked Turtle moved to stick his head through the portal. The eyes of his three brothers widened.

"Donny, what the shell are ya doin'?" Raph demanded.

"Relax," Don replied. "I've got it under contro-aaah!"

"DONNY!"

Donatello had tripped on a loose wire, and was falling through the portal. Mikey grabbed for his brother, but Leo and Raph pulled him back as Don disappeared into the portal. Once he was through, the stick whirred, and the portal faded as the machine shut off. Mikey's blue eyes were wide.

"Donny? Donny! DONNY!"